In general, as the low-pass filter, one having a basic arrangement as shown in FIG. 1 has been known, in which inductances L.sub.1, L.sub.2, etc each disposed in series are grounded via capacitances C.sub.E1, C.sub.E3, C.sub.E3, etc.
In addition, as a low-pass filter having an attenuating pole formed in the neighborhood of the cut-off frequency for achieving a steep attenuating characteristic, as shown in FIG. 2, one having an arrangement using a parallel connection of a capacitor C.sub.1 and a coil L.sub.1 and a parallel connection of a capacitor C.sub.2 and a coil L.sub.2 has been known.
With such a low-pass filter, a stray capacitance as indicated by broken line in FIG. 2 is generated to the LC parallel connection due to the arrangement of the used coil. This stray capacitance is substantially, difficult to remove, and has a considerable distribution. This distribution in turn causes a distribution of the resonant frequency of the LC parallel connection or of the impedance in the frequency pass-band ultimately affecting the filter frequency characteristic.
This effect, although small when the frequency is low, becomes greater if the frequency is high thus causing the fluctuation of the attenuating pole frequency and the cut-off frequency or the increase of the mismatching loss in the frequency passband.
Therefore, unless a considerable adjustment is made to the coil or capacitor, any desired filter frequency characteristic cannot be obtained, and it is complicated and difficult to adjust the filter frequency characteristic.
In addition, in general, as the high-pass filter, one having a basic arrangement as shown in FIG. 3 has been known, in which capacitances C.sub.1, C.sub.2, etc each disposed in series are grounded via inductances L.sub.E1, L.sub.E2, L.sub.E3 and the like.
In addition, as the high-pass filter having an attenuating pole formed in the neighborhood of the cut-off frequency for achieving a steep attenuating characteristic, as shown in FIG. 4, one having an arrangement using a parallel connection of the capacitor C.sub.1 and the coil L.sub.1 and a parallel connection of the capacitor C.sub.2 and the coil L.sub.2 has been known.
However, such a high-pass filter also suffers from a similar problem as in the aforementioned low-pass filter and, unless a considerable adjustment is made to the coil or capacitor, a desired filter frequency characteristic cannot be achieved, and it is complicated or difficult to adjust the filter frequency characteristic.
Further, in general, as the band-pass filter, one having a basic arrangement as shown in FIG. 5 has been known, in which capacitances C.sub.1, C.sub.2, C.sub.3, C.sub.4, etc and inductances L.sub.1, L.sub.2, L.sub.3, L.sub.4, etc each alternately disposed in series are grounded via capacitances C.sub.E1, C.sub.E2, C.sub.E3, etc.
Still further, as the band-pass filter having an attenuating pole formed in the neighborhood of the frequency passband for achieving a steep attenuating characteristic, as shown in FIG. 6, one having an arrangement using a parallel connection of a capacitor C.sub.F1 and a coil L.sub.1, a parallel connection of a capacitor C.sub.F2 and a coil L.sub.2, a parallel connection of a capacitor C.sub.F3 and a coil L.sub.3, a parallel connection of a capacitor C.sub.F4 and a coil L.sub.4 and the like has been known.
Such a band-pass filter also suffers from a similar problem as in the aforementioned low-pass filter or high-pass filter and, unless a considerable adjustment is made to the coil or capacitor, no desired filter frequency characteristic is obtained and, it is complicated and difficult to adjust the filter frequency characteristic.
Thus, it is proposed to use a .lambda./4 coaxial dielectric resonator using a dielectric material having a high dielectric constant in order to form a band-pass filter of high frequency range. The arrangement of a conventional band-pass filter using the dielectric resonator is illustrated in FIG. 7, in which 1A', 1B' and 1C' each denote a dielectric resonator, whose outer conductor is grounded. However, according to this arrangement, it is not possible to form the attenuating pole in the neighborhood of the upper or lower limit of the frequency passband to achieve the steep attenuating characteristic while, as the number of stages is increased, the insertion loss can be greatly increased.